Saturday, September 4, 2010

Altitude brake

My Dobsonians move very smoothly.  The problem with that is that if I am showing things to the public, it is easy for people to push the scope away from the object, especially in altitude.  Before the last public star party, I made a little altitude brake, based on ideas on Cloudy Nights.  I used the router to cut a plywood disc the size of the altitude bearings (7" diameter) on my 13" scope.  (Actually, one could just cut a triangular piece with a hand saw.)  A plywood strip (with a plywood pad on the other end) is glued to it as a handle.  I attached three grippy rubber pads (one could put silicone sealant pads, too) near the circumference.  A bolt keeps the handle immobile, and a big 1/4"-20 carriage bolt runs from the inside of the mirror box, through the altitude bearing, through the disc, and then there is a fender washer and a knob to adjust tension.  The handle is bolted to the rocker box.  At low tension, movement is slowed down, but still smooth, despite the grippy pads.  At full tension, movement is stopped.  The original design I was following used a spring, but that doesn't seem to do anything.

I also made a smaller 2.5" version with Velcro hook pads just to slow down, but not stop, movement.  I didn't try it in the field, though.

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